Selective catalytic reduction (“SCR”) exhaust aftertreatment systems are an important technology for reducing NOx emissions from internal combustion engines such as diesel engines. SCR systems generally include a source of reagent such as a urea solution, a dosing system for pressurizing the urea solution and for providing a controlled amount or rate of urea solution to an SCR catalyst, and an injector which provides urea solution to a urea decomposition region of an exhaust flowpath located upstream from an SCR catalyst. Many SCR systems also utilize pressurized gas to assist the flow of urea solution to the injector.
While providing important reductions in NOx emissions, SCR systems suffer from a number of shortcomings and problems. Use of urea solutions in SCR systems may result in growth of urea crystals or deposits on various components of the system which may disrupt their operation. Injector nozzles may become blocked due to, for example, formation of urea deposits when urea solution is exposed to elevated temperatures. One or more portions of the system may be crimped, blocked or non-functional, either intentionally or unintentionally. These and other conditions result of the dosing system in insufficient reagent being injected into the exhaust stream for the treatment of NOx emissions by the SCR catalyst.
Existing techniques have been developed to monitor urea usage in SCR systems and predict or determine a fault condition of the dosing system. One technique compares the volume of reagent commanded over a time period by integrating the commanded reagent flow rate over the time period, and then comparing this commanded volume to the volume of reagent consumed from the storage tank. However, these techniques are not reliable and are prone to indicating false failures due to many factors which cannot be controlled by the supplier of the dosing system, such as operator re-fill behavior, reagent tank geometry, reagent fluid level sensor resolution, and reagent tank overfill volume. Another technique involves placing a flow meter and associated sensing hardware to provide a real time measurement of the flow rate, which is then compared to the commanded flow rate. However, this techniques requires additional expense in the dosing system both in terms of initial hardware cost and maintenance and repair costs over time. There is a long felt need for advancements mitigating these and other shortcomings associated with monitoring and diagnosing faults in reagent dosing systems.